Compositions and methods for coating surfaces

ABSTRACT

An exterior building cladding system and methods of applying the building cladding system are described. The system is a weather resistive barrier and decorative system that is particularly effective as a covering system for buildings having an exterior wood substrate. The system includes a flexible waterproof base layer of acrylic and rubber that is adhered directly to a wood substrate, a mesh embedded in the flexible waterproof base layer and an acrylic stucco outer layer adhering directly to the flexible waterproof base layer. The system eliminates the need for building wrap while providing a moisture resistant and vapour permeable layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Canadian Patent Application No. ______ filed on Aug. 13, 2009, entitled “Compositions and Methods for Coating Surfaces”, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

An exterior building cladding system and methods of applying the building cladding system are described. The system is a weather resistive barrier and decorative system that is particularly effective as a covering system for buildings having an exterior wood substrate. The system includes a flexible waterproof base layer of acrylic and rubber that is adhered directly to a wood substrate, a mesh embedded in the flexible waterproof base layer and an acrylic stucco outer layer adhering directly to the flexible waterproof base layer. The system eliminates the need for building wrap while providing a moisture resistant and vapour permeable layer.

BACKGROUND OF THE INVENTION

The use of stucco as a construction material for exterior cladding on commercial and residential buildings is well known in the prior art. Stucco cladding generally consists of a mixture of Portland cement, sand, water, and sometimes small quantities of lime. Stucco is applied to a building exterior in a wet plastic state and allowed to dry to form a rough, durable exterior cladding. Generally, two or more coats of stucco are applied to a building exterior, including a base coat and a top coat. Stucco can be applied directly to various surfaces including brick or stone surfaces.

As is well known, wood frame construction is common for residential and commercial buildings in North America. Wood frame construction generally incorporates exterior sheathing over the wood frame including Oriented Strand Board (OSB), plywood, or particle board. As stucco does not readily adhere directly to wood substrates, a mesh or lath is generally attached to the exterior sheathing beneath the stucco layers to give the stucco an anchor to attach to. As such, the mesh layer creates an additional layer that must be applied as part of the exterior cladding process.

As stucco cladding is quite rigid and brittle, it is susceptible to cracking due to shifts in a building frame and/or foundation due to natural settling, and/or due to expansion or contraction of building components caused by temperature and humidity changes. Being made from cement and sand, stucco cladding is quite porous and is easily penetrated by water, especially if cracks are present in the stucco.

Modern stucco has addressed some of the problems of traditional stucco by various means including incorporating acrylic-based finish coats over a cementitious base coat that are provided to reduce the risk of exterior surface cracking. However, surface cracking in acrylic-based finish coats is not completely eliminated and water penetration is still possible.

As water penetration can lead to costly water damage to the building cavity, modern buildings also include a vapour-permeable, water-resistant barrier wrapped around the entire exterior building surface beneath the stucco layer. Traditionally, asphalt-saturated paper was used as the weather barrier, but today there are numerous manufactured plastic-based sheets in the market for this use, generally referred to as “building wraps” or “building membranes”. Such building wraps are described in U.S. Pat. No. 7,148,160 and U.S. Pat. No. 6,355,333.

As with wood surfaces, stucco does not readily adhere to building wraps, with the result being that a mesh or lath is generally required to be fastened to the exterior of the building wrap by means of fastening devices such as nails or staples. The use of fastening devices creates small puncture holes in the building wrap that increases the susceptibility of the building wrap to water leakage, defeating the purpose of the building wrap. If water does get underneath the building wrap, it often becomes trapped between the building wrap and the underlying substrate, which can lead to mold and mildew problems that can damage the building structure and lead to other problems. As such, drainage mats are sometimes used between the building wrap and the underlying substrate to create space and a drainage path for trapped moisture to exit the wall. This creates yet another layer that must be applied in an exterior cladding process.

As a result, the application of stucco as an exterior cladding on wood substrate structures can be a time-consuming, labor-intensive process due to the myriad of layers that are required to prevent moisture penetration into the building structure. In some constructions, there may be as many as five layers applied individually, including a building wrap, mesh or lath, drainage mat, stucco base coat, and stucco top coat.

Certain approaches to decrease time and labor costs for the application of exterior cladding systems involve the use of pre-fabricated composite material products that combine one or more necessary layers into one product that can be installed as a unit. U.S. Patent Application No. 2007/0051069 describes a composite building material that includes a building membrane, a spacer material for drainage, and a lath bonded into one unit. U.S. Pat. No. 6,131,353 discloses a composite drainage mat with a weather barrier attached. U.S. Patent Application No. 2006/0101758 describes a corrosion-resistant non-metal lath attached to a weather barrier. Pre-fabricated cementitious fiber wallboard panels coated with a water based acrylic are described in U.S. Pat. No. 6,516,580.

As pre-fabricated composite systems still consist of the typical exterior cladding layers, there is still the same amount of material in the composite products as there would be if the layers came as individual units. This generally results in pre-fabricated rolls or sheets of composite products that are bulky, heavy, and awkward to install, requiring a large labor component for their installation. Once they are installed, the numerous joints between the sections must be sealed and/or the product must be installed with sufficient overlap between the sections to decrease the chance of water penetration at the joint, however this increases the amount of product that must be used. As well, the pre-fabricated products are susceptible to damage during the transportation and handling of the products prior to installation.

Regardless of the shortcomings of stucco, it is still a very popular exterior cladding material due to its attractiveness, wide variety of color options, durability, and low price. A variety of products are available to decrease the susceptibility of stucco cladding to water penetration, however a review of the prior art shows there is still a need for an exterior stucco cladding system for wood substrate buildings that is simple and quick to install and prevents water penetration into the underlying wood substrate while still allowing water vapor to escape from the building structure.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided an exterior building cladding system and method for cladding a building having an exterior surface.

More specifically, there is provided an exterior building cladding system for application to a building substrate comprising:

-   -   a flexible waterproof and water vapor permeable base layer         composition of acrylic and rubber for adhering directly to the         building substrate;     -   a reinforcement mesh for embedding in the flexible waterproof         and water vapor permeable base layer composition prior to curing         of the flexible waterproof and water vapor permeable base layer         composition; and     -   an acrylic stucco outer composition for adhering directly to the         flexible waterproof and water vapor permeable base layer         composition.

In one embodiment of the invention, the system includes a joint sealing system for sealing joints in the building substrate, the joint sealing system comprising:

-   -   a curable acrylic and cementitious joint sealing composition for         adhering directly to joints within the building substrate; and     -   a joint reinforcement mesh for embedding in the joint sealing         layer prior to curing.

Preferably, the reinforcement mesh and joint reinforcement mesh are made of fiberglass.

In another embodiment, the flexible waterproof and water vapor permeable base layer includes rubber crumb from recycled tires of about size 40 mesh.

Preferably, the flexible waterproof and water vapor permeable base layer composition includes 55-65% solids by weight, including by weight: 18-23% rubber crumb; 1-2% TiO₂; 2-3% hard resin acrylic solids; 12-13% soft resin acrylic solids; and 25-30% CaCO₃.

In further embodiments, the flexible waterproof and water vapor permeable base layer will have any or all of the following characteristics: a water vapor permeability rate of greater than 60 ng/Pa.s.m²; impermeability to liquid water for up to 1.75 hr exposure to liquid water; a density of 1.25-1.35 kg/Litre; a viscosity of 350-600 KcP before curing; a bond strength to plywood and oriented strand board (OSB) of greater than 5 psi after 2000 hours of weathering; a tensile strength of greater than 20 lbs/inch; fire resistance; and/or a smoke developed classification of less than 20.

In yet another embodiment, the flexible waterproof and water vapor permeable base layer composition includes 35-45% by weight volatile compounds.

In another embodiment, the flexible waterproof and water vapor permeable base layer composition includes about 17.1 g/Litre of volatile organic compounds before curing.

In a preferred embodiment, the flexible waterproof and water vapor permeable base layer composition is applied to a building substrate with about a film thickness of 2 millimeters.

In one embodiment, the joint sealing layer composition includes acrylic and Portland cement at about a 1:1 by weight ratio.

In further embodiments, the cured joint sealing layer composition will have any one of or a combination of the following characteristics: an air leakage value of less than 0.01 L/sec/m²; a coefficient of water absorption of 0.0009 or less; and/or a bond strength to concrete or OSB of greater than 0.1 MPa after 2 hours and greater than 0.3 MPa after 7 hours.

In another aspect of the invention, there is provided a method of cladding a building having an exterior surface comprising the steps of: applying a flexible waterproof and water vapor permeable base layer to the exterior surface; embedding a reinforcement mesh in the flexible waterproof and water vapor permeable base layer; allowing the flexible waterproof and water vapor permeable base layer to cure to form a cured base layer; and applying an acrylic stucco outer layer to the cured base layer.

In another embodiment, the method further comprises applying a joint sealing layer embedded with a joint reinforcement mesh to joints in the exterior surface prior to step a).

In accordance with the method, the reinforcement mesh and joint reinforcement mesh may be fiberglass.

In accordance with the method, the flexible waterproof and water vapor permeable base layer and the joint sealing layer may have a composition as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the accompanying FIGURE in which the FIGURE is a perspective view of a layered exterior cladding system in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Overview

With reference to the FIGURE, a layered exterior cladding system (ECS) 10 is described.

As shown in FIG. 1, the ECS 10 is preferably applied directly to a frame constructed wall with a wood-based substrate. As shown, the typical wood frame wall includes evenly spaced structural members 12 and exterior panels or sheathing 14. The ECS can also be applied to non-wood substrates, such as cementitious-based substrates and insulated concrete forms (ICF's) made of expanded polystyrene foam (EPS), with additional surface preparation. Importantly, the subject system does not require the use of metal fasteners and, as explained in greater detail below, creates a watertight surface in which the underlying sheathing is not compromised by penetrating fasteners. In addition, the system eliminates the need for house wrap and wire meshes.

In accordance with the invention, the ECS includes a joint sealing system 16 that is comprised of a first mesh layer 18, a joint compound 20, and a wall coating system 22 that includes a second mesh layer 24, a flexible rubber basecoat 26, and an acrylic stucco finish coat 28.

Joint Sealing System

The joint sealing system 16 is designed to seal all the joints, seams, holes, and cracks in the frame constructed wall from moisture penetration.

First Mesh Layer

The first mesh layer 18 is preferably made of fiberglass and is designed to provide increased surface strength, flexibility, as well as impact and crack resistance in the ECS. The first mesh layer is typically manufactured in 8 inch (20.3 cm) wide rolls and cut to the required length at the job site. Table 1 shows desired properties and test values for the first mesh layer 18 and second mesh layer 24.

TABLE 1 Properties and Test Values for Reinforcing Mesh Layers 18, 24. Property Test Method Units Test Value Construction ASTM D3775 Warp (/in) 6.0 Weft (/in) 5.0 Weight ASTM D3776 Oz/yd² 4.3 Thickness ASTM D1777 /in 0.017 Weave — — Leno Finish — — Alkali Resistant Nominal Tensile ASTM D5035 Warp (lb/in) 185 Weft (lb/in) 220

Joint Compound

The joint compound 20 is a specially formulated acrylic-based compound that is flexible and can expand and contract with changes in the frame constructed wall while still retaining a seal between the joints. The joint compound is preferably manufactured as a plastic acrylic compound that is mixed on the job site with clean type 10 Portland cement in a 1:1 ratio by weight, together with small amounts of water (if any) to provide a desired workability. Preferably, an approximately 2 mm ( 5/64 inch) thick layer of the mixed joint compound is applied within 2-3 hours to the joints and seams of the wall by means of a trowel, with the joints and seams preferably no larger than 6.35 mm (¼ inch) wide. The first mesh layer 18 is pressed into the wet joint compound and the joint compound is feathered out smoothly over the first mesh layer and allowed to set. The initial setting time of the acrylic resin in the joint compound is generally 24 hours and the cement in the compound generally fully cures in about 28 days. Generally, the ambient temperature must be 5° C. (41° F.) or greater during the application and curing process. Typically, only one coat of joint compound is needed. However, if a second coat of joint compound is needed to completely cover the first layer of mesh 10, the second coat can be applied after the initial setting time of approximately 24 hours. Desired properties and test data for the joint compound is provided in Table 2.

TABLE 2 Performance Properties of Joint Compound 20 Property Test Method Test Criteria Test Result Air Leakage ASTM E283 <0.02 L/sec/m² <0.01 L/sec/m² Coefficient of CEN-TC89 <0.004 0.0009 Water Absorption Bond Strength to UEAct Directive 2-hour >0.1 MPa Pass Concrete Section 3.2.1.3 7-hour >0.3 MPa Pass Bond Strength to UEAct Directive 2-hour >0.1 MPa Pass OSB Section 3.2.1.3 7-hour >0.3 MPa Pass Bond Strength to UEAct Directive 2-hour >0.1 MPa Pass Basecoat Section 3.2.1.3 7-hour >0.3 MPa Pass

Wall Coating System

Second Mesh Layer

The second mesh layer 24 is normally the same material (fiberglass) as the first mesh layer 10 and serves the same function: to increase the surface strength, flexibility, and impact and crack resistance of the ECS. Table 1 outlines the technical data for the mesh layers. The second mesh layer is preferably manufactured in rolls 38 inches wide (96.5 cm) and cut to length at the job site. When applied, as described below, the edges of adjacent mesh layers preferably overlap a minimum of 2 inches (5.1 cm).

Flexible Basecoat

The flexible rubber basecoat 26 is made of an acrylic polymer impregnated with rubber crumb granules, preferably of about size 40 mesh, and preferably from recycled tires. Technical data and testing data for the basecoat is outlined in Table 3 and Table 4 below. The basecoat adheres directly to the wood substrate exterior panels of the building and provides a flexible waterproof coating in the building envelope. The basecoat is formulated to allow for the transmission of water vapor at a rate greater than 60 ng/Pa.s.m² at the recommended thickness (dry) of 1-1.4 mm ( 1/32- 3/64 inch) such that the basecoat when cured is permeable to vapor water molecules yet impermeable to liquid water molecules. This allows for the building frame to be protected from water damage, yet prevents moisture from being trapped in the building. As the basecoat is an elastic polymer, it is flexible and is less susceptible to cracking as the building shifts or expands and contracts from thermal stresses.

TABLE 3 Properties of Flexible Rubber Basecoat Property Value Solids by weight (wt %) 55-65 TiO₂ (wt %) 1-2 Rubber Crumb by weight (wt %) 18-23 Acrylic Solids (wt %) Hard Resin (Minimum Film Forming 2-3 Temperature (MFFT) 15-20° C.) Soft Resin (MFFT <1° C.) 12-13 CaCO₃ (wt %) 25-30 Volatile Compounds (wt %) 35-45 Volatile Organic Compounds (g/Litre) 17.1 pH 8.0-9.0 Density (kg/Litre) 1.25-1.35 Required Viscosity (KcP) 350-600 Required Application Film Thickness (mm) 2 

TABLE 4 Performance Properties of Flexible Rubber Basecoat Property Test Method Test Criteria Test Result Water Vapour ASTM E96 >60 ng/Pa · s · m² Pass Transmission Accelerated ASTM G23-81 2000 hours of No deleterious Weathering exposure effects Bond Strength after ICBO ES AC59 V.E >5 psi Pass Weathering to OSB & Plywood Freeze-Thaw ICBO ES AC59 V.D 10 freeze-thaw No deleterious cycles effects Salt-Spray ASTM B117-85 300 hours of No deleterious Resistance exposure effects Pliability CAN/CGSB 51-32- No cracking @ −10° C. Pass M77 para. 5.2 Tensile Strength CAN/CGSB 51-32- >20 lbs/inch Pass M77 para. 5.3 Water Resistance ASTM D 779-94 Recorded No permeance after 1 hr. 45 min. Flamespread CAN/ULC S102-07 Recorded 0 Flamespread Value (FSV) Smoke CAN/ULC S102-07 Recorded Smoke <20 Development Developed Classification (SDC)

The flexible rubber basecoat is applied in a plastic state, tinted or untinted, over the entire clean building surface and over the set joint compound in a layer thick enough to completely embed the second mesh layer 24 that is pressed into the basecoat immediately after the application of the basecoat. The basecoat is troweled smooth over the second mesh layer and allowed to cure. Upon exposure to the atmosphere, the basecoat dries in approximately 24 hours in ideal conditions (21° C./70° F. at 50% R.H.) by water evaporation and cures by the coalescence of the acrylic polymer in the basecoat to form a continuous film. During the application and curing of the basecoat the ambient temperature should be >5° C. (41° F.) and the basecoat should be protected from rain. Only one layer of basecoat is necessary, however a second layer can be applied after the first layer has dried (approximately 24 hours) if needed.

Upon setting, the basecoat creates a continuous seamless waterproof surface over the coated surface and with minimal maintenance will generally last the life of the building. The second mesh layer in the basecoat acts as a support structure for the rubber basecoat, much like the steel belt in a rubber tire supports a rubber tire tread. As known to those skilled in the art appropriate flashing, termination strips, corner beads and/or other detailing structures can be incorporated in and around window and door openings and corners.

Acrylic Stucco Finish Coat

The acrylic stucco finish coat 28 is a typical stucco containing acrylic that is well known to those versed in the art. In the preferred embodiment, the finish coat is tinted to any color and is of fine, medium, or coarse texture. The finish coat is troweled directly on top of the rubber basecoat 26 (after the basecoat has cured) in a thickness just greater than the larger aggregate in the finish coat. The finish coat generally dries in 24 hours and with minimal maintenance will generally last the life of the building. 

1. An exterior building cladding system for application to a building substrate comprising: a flexible waterproof and water vapor permeable base layer composition of acrylic and rubber for adhering directly to the building substrate; a reinforcement mesh for embedding in the flexible waterproof and water vapor permeable base layer composition prior to curing of the flexible waterproof and water vapor permeable base layer composition; and an acrylic stucco outer composition for adhering directly to the flexible waterproof and water vapor permeable base layer composition.
 2. A system as in claim 1 further comprising a joint sealing system for sealing joints in the building substrate, the joint sealing system comprising: a curable acrylic and cementitious joint sealing composition for adhering directly to joints within the building substrate; and a joint reinforcement mesh for embedding in the joint sealing layer prior to curing.
 3. A system as in claim 1 wherein the reinforcement mesh is a fiberglass mesh.
 4. A system as in claim 2 wherein the joint reinforcement mesh is a fiberglass mesh.
 5. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition includes rubber crumb.
 6. A system as in claim 5 wherein the rubber crumb is from recycled tires.
 7. A system as in claim 5 wherein the rubber crumb size is about 40 mesh.
 8. A system as in claim 5 wherein the rubber crumb is 18-23% by weight of the flexible waterproof and water vapor permeable base layer composition.
 9. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition has a water vapor permeability rate of greater than 60 ng/Pa.s.m².
 10. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition is impermeable to liquid water for up to 1.75 hr exposure to liquid water.
 11. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition includes 55-65% solids by weight.
 12. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition includes 1-2% by weight TiO₂.
 13. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition includes 2-3% by weight hard resin acrylic solids.
 14. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition includes 12-13% by weight soft resin acrylic solids.
 15. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition includes 25-30% by weight CaCO₃.
 16. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition includes 35-45% by weight volatile compounds.
 17. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition has a density of 1.25-1.35 kg/Litre.
 18. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition includes about 17.1 g/Litre of volatile organic compounds before curing.
 19. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition has a viscosity of 350-600 KcP before curing.
 20. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition is applied to a building substrate with a film thickness of about 2 millimeters.
 21. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition has a bond strength to plywood and oriented strand board (OSB) of greater than 5 psi after 2000 hours of weathering.
 22. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition has a tensile strength of greater than 20 lbs/inch.
 23. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition is fire resistant.
 24. A system as in claim 1 wherein the flexible waterproof and water vapor permeable base layer composition has a smoke developed classification of less than
 20. 25. A system as in claim 2 wherein the joint sealing layer composition includes acrylic and Portland cement at about a 1:1 by weight ratio.
 26. A system as in claim 2 wherein a cured joint sealing layer composition has an air leakage value of less than 0.01 L/sec/m².
 27. A system as in claim 2 wherein a cured joint sealing layer composition has a coefficient of water absorption of less than 0.0009.
 28. A system as in claim 2 wherein a cured joint sealing layer composition has a bond strength to concrete of greater than 0.1 MPa after 2 hours and greater than 0.3 MPa after 7 hours.
 29. A system as in claim 2 wherein a cured joint sealing layer has a bond strength to oriented strand board (OSB) of greater than 0.1 MPa after 2 hours and greater than 0.3 MPa after 7 hours.
 30. A system as in claim 2 wherein a cured joint sealing layer composition has a bond strength to a cured flexible waterproof and water vapor permeable base layer composition of greater than 0.1 MPa after 2 hours and greater than 0.3 MPa after 7 hours.
 31. An exterior building cladding system for application to a building substrate comprising: a flexible waterproof and water vapor permeable base layer composition of acrylic and rubber for adhering directly to the building substrate, comprising by weight 18-23% rubber crumb of about size 40 mesh from recycled tires; a fiberglass reinforcement mesh for embedding in the flexible waterproof and water vapor permeable base layer composition prior to curing of the flexible waterproof and water vapor permeable base layer composition; and an acrylic stucco outer composition for adhering directly to the flexible waterproof and water vapor permeable base layer composition; wherein the base layer has a water vapor permeability rate of greater than 60 ng/Pa.s.m².
 32. A system as in claim 31 further comprising a joint sealing system for sealing joints in the building substrate, the joint sealing system comprising: a curable acrylic and cementitious joint sealing composition for adhering directly to joints within the building substrate, including acrylic and Portland cement at about a 1:1 by weight ratio; and a fiberglass joint reinforcement mesh for embedding in the joint sealing layer prior to curing; wherein a cured joint sealing layer composition has an air leakage value of less than 0.01 L/sec/m², a coefficient of water absorption of less than 0.0009 and a bond strength to concrete, oriented strand board (OSB) and a cured flexible waterproof and water vapor permeable base layer composition of greater than 0.1 MPa after 2 hours and greater than 0.3 MPa after 7 hours.
 33. A method of cladding a building having an exterior surface comprising the steps of: a) applying a flexible waterproof and water vapor permeable base layer to the exterior surface; b) embedding a reinforcement mesh in the flexible waterproof and water vapor permeable base layer; c) allowing the flexible waterproof and water vapor permeable base layer to cure to form a cured base layer; and d) applying an acrylic stucco outer layer to the cured base layer.
 34. A method as in claim 33 further comprising applying a joint sealing layer embedded with a joint reinforcement mesh to joints in the exterior surface prior to step a).
 35. A method as in claim 33 wherein the base layer in step a) has a composition comprising by weight 18-23% rubber crumb of about size 40 mesh from recycled tires, the reinforcement mesh in step b) is fiberglass and the cured base layer in step c) has a water vapor permeability rate of greater than 60 ng/Pa.s.m².
 36. A method as in claim 35 wherein the joint sealing layer is curable and includes acrylic and Portland cement at about a 1:1 by weight ratio, the joint reinforcement mesh is fiberglass and the cured joint sealing layer has an air leakage value of less than 0.01 L/sec/m², a coefficient of water absorption of less than 0.0009 and a bond strength to concrete, oriented strand board (OSB) and the cured base layer of greater than 0.1 MPa after 2 hours and greater than 0.3 MPa after 7 hours. 